feat(KB-501): implement multi-project runtime architecture with HybridExecutor

- Add ProjectRuntime interface with InProcessRuntime and ChildProcessRuntime implementations
- Implement HybridExecutor for multi-project task orchestration
- Add IPC protocol for host-worker communication in child-process mode
- Implement health monitoring with automatic restart for child processes
- Add comprehensive tests for hybrid-executor and project-runtime
- Update AGENTS.md with multi-project runtime documentation
- Add changeset for hybrid-executor-runtime release

packages/engine/src/logger.ts

@@ -73,3 +73,6 @@ export const ipcLog = createLogger("ipc");
 
 /** Logger for the project manager subsystem. */
 export const projectManagerLog = createLogger("project-manager");
+
+/** Logger for the hybrid executor subsystem. */
+export const hybridExecutorLog = createLogger("hybrid-executor");